Morphological and physiological differences in Synechococcus elongatus during continuous cultivation at high iron, low iron, and iron deficient medium

Thermophilic unicellular cyanobacterium Synechococcus elongatus Nag. var. t hermalis Geitl. strain Kovrov 1972/8 was cultivated in continuous flow reac tor to simulate conditions occurring in nature in regions with low iron con centration. Two degrees of iron deprivation were established: (a) low iron (LI) conditions (9.0 muM Fe) when cells still maintained maximal growth rat e but already exhibited changes in photosynthetic apparatus, and (b) iron d eficient (ID) conditions (0.9 muM Fe) when cell growth rate decreased and e xtensive morphological and functional changes were observed. A decrease in the cellular content of phycobilin antenna was observed in both ID and LI c ells and an increase of carotenoid concentration only in the ID culture. Mo rphologically, ID cells showed a decrease in the amount of phycobilins and in the number of thylakoid membranes. This suggests that S. elongatus respo nds to decrease in iron availability by substitution of the phycobilisomes by antennae containing chlorophyll (Chl) and carotenoids. Photochemical act ivity of photosystem (PS) 2, determined as F-v/F-m, ratio was similar in hi gh iron (HI) and LI cultures and approximately five times lower in ID cultu re. On the other hand, the activity of the whole electron transport chain s howed the opposite tendency: the relative rates of the CO2-dependent oxygen evolution in HI:LI:ID cultures were approximately 1 :2 :4. Thus in nutrien t stress the photosynthetic apparatus preserved its activity despite the de crease in the amount of both Chi-binding complexes and thylakoid membranes.